This invention relates to a gear shift control apparatus for use with an automatic transmission having a friction element changeable into an engaged state to effect a change to a lower gear in response to a high level of hydraulic pressure applied thereto.
For example, Japanese Patent Kokai No. 6-11030 discloses a gear shift control apparatus arranged to monitor the gear ratio provided in an automatic transmission to effect a change from a higher gear to a lower gear. The automatic transmission includes a low clutch operable on a working fluid having a changeable pressure. When the gear ratio exceeds a reference value, the working fluid pressure is changed to its high level causing the low clutch to engage so as to produce a change from forth to third gear. It is proposed, in this Japanese Patent Kokai, to produce such a change down at an appropriate time regardless of the working fluid temperature by increasing the reference gear ratio value as the working fluid temperature increases.
The conventional gear shift control apparatus determines the reference gear ratio value at which the working fluid pressure is changed to its high level in dependent on the engine speed and the vehicle speed. Assuming now that the reference gear ratio value is set for a middle vehicle speed, the time at which the low clutch is engaged will be retarded when a change is effected from fourth to third gear at a low vehicle speed. In this case, the low clutch is engaged after completion of the change from fourth to third gear to provide a bad feel because of engine racing during the gear shift operation. On the other hand, the time at which the low clutch is engaged will be advanced when a change is effected from fourth to third gear at a high vehicle speed. In this case, the low clutch is engaged before completion of the change from fourth to third gear to produce shocks because of interlock during the gear shift operation.